Toothbrushes are typically manufactured using an injection molding process. Such an injection molding process is characterized by providing a mold in the shape of the toothbrush and injecting molten plastic through a hot channel nozzle into the mold. The toothbrush is then cooled and ejected from the mold. For example, U.S. Pat. No. 5,845,358 shows such a toothbrush made by injection molding. One of the limitations of the conventional injection molding processes is that large diameter handles, and especially large handles having substantial contours along their length cannot be produced in an efficient manner.
Toothbrushes with increased handle diameters provide substantial advantages, for instance they can provide increased gripping area for children increasing the ability of children to handle and use toothbrushes; also people with disabilities such as arthritis sometimes have difficulty in handling toothbrushes due to difficulty in flexing the joints in their hands. Such difficulties are considerably relieved by means of toothbrushes having increased handle diameters. Additionally, the larger cross section handles on the toothbrushes are better for the user from an ergonomic point of view.
Even better from an ergonomic point of view are large handles having contours, characterized by one or more substantial variations in cross sectional area, including both increasing and decreasing cross sectional area, along the length or major axis of the brush. Such variation in cross-sectional area allows the user to have better grip and handling of the brush during use, when it must be rapidly moved, often while wet or slippery.
Even though there are advantages to toothbrushes having increased handle diameters, the use of injection molding to manufacture toothbrushes with larger cross section handles has at least five drawbacks.
First—the toothbrush is more expensive due to the use of more plastic to make the toothbrush. Since toothbrushes made in Injection Molding are solid, the material used to create the toothbrush handle increases approximately with the square of the diameter of the handle.
Second—the cost of manufacture is increased because the time needed to cool and solidify the toothbrush increases considerably with increasing diameter. The increased cooling time is due both to the increased quantity of hot plastic, and the larger cross section of the toothbrush. As plastic has a relatively low thermal conductivity, extracting heat from the center of the brush is substantially more difficult with an increased cross section.
Third—most thermoplastics shrink during cooling and solidification. Shrinkage can be mitigated by packing additional molten plastic into the center of the handle through the injection gate as the outer edges of the handle cool. However this mitigation loses effectiveness as the injection gate is placed away from the thickest portion of the handle, and placement of the gate, which will have some tactile vestige, in the thick, gripping portion of the handle, can lead to dissatisfaction during use. For many toothbrush handle designs, packing alone cannot mitigate the visible surface shrinkage and surface and internal defects associated with an increased handle cross section. These surface defects can be manifested as unintentional variations in surface gloss or texture, which contribute negatively to the look and feel of the part. Internal defects can be manifested as voids or bubbles inside the plastic, which can weaken the handle visibly or invisibly, depending on the degree of transparency of the plastic.
Fourth—the injection molding process requires sufficient energy to fully melt the plastic to a liquid state, so that it can travel under pressure through the runner, nozzle, and gate to completely fill the injection mold cavity.
Fifth—the filling and packing of the plastic into the injection mold cavity requires very high pressures, typically ranging from thousands of pounds per square inch to tens of thousands of PSI, which necessitates mold cavities made from very high-strength materials such as hard steel, which are expensive and time-consuming to create. Further, the very high fluid pressure of the plastic onto the mold cavity surfaces is non-uniform across the cavity, leading to non-uniform wear of the tooling cavity, which ultimately will affect cosmetic features in the molded plastic part.
In an attempt to overcome the difficulties associated with the use of injection molding to produce toothbrush handles having increased diameters, it has been suggested to produce toothbrush handles having a hollow body. For example, EP 0 668 140 or EP 0 721 832 disclose the use of air assist or gas assist technology to make toothbrushes having hollow, large cross-sectional handles. In the disclosed process, molten plastic is injected near the base of the toothbrush handle, wherein subsequently a hot needle is inserted into the molten plastic to blow gas into the molten plastic which is then expanded towards the walls of the injection mold. In a similar manner, U.S. Pat. No. 6,818,174 B2 suggests injecting a predetermined amount of molten plastic into the cavity to only partially fill the mold cavity and subsequently inject a gas through a gas injection port formed in the injection mold to force the molten plastic into contact with the walls of the mold cavity. CN102166064 discloses a hollow toothbrush handle and a method of toothbrush production, in particular toothbrush handle production. When the molten plastic material is injected into the mold cavity of the brush handle, any position of the brush handle is provided with a blow hole, gas is blown into the center of the brush handle through the blow hole, and the blow hole is sealed after the brush handle is shaped. The described toothbrush descried has a hollow handle and solid toothbrush head made through a gas-assisted injection molding process.
To produce void spaces in the molten plastic such injection molding processes inject air at very high pressures, often greater than 1,000s PSI, making it difficult to form hollow handle bodies with substantially uniform wall thickness or with an overmolded second or third shot. The wall thickness of the handle made in this method is not evenly distributed, resulting in less optimal weight distribution for better handle strength. The amount of material saved is also limited to 10˜50% handle weight. As such, the potential for material reduction and increased efficiency in manufacturing are limited.
A conventional method to create toothbrush handles having increased cross sections, such as electromechanical toothbrush handles, is to manufacture discrete parts of the handle separately using injection molding, then to assemble these parts in either a separate non-injection molding step, or in a subsequent injection molding step whereby the discrete parts from the first step or steps are inserted into an injection mold first and one or more additional materials are injected around them, creating a hollow body from multiple parts. This manufacturing method still has the drawbacks of: requiring the complete melting of plastic, high pressures, associated equipment involved with injection molding, and in addition may have added labor expense associated with both in-mold and out-of-mold assembly.
In the assembly of a blow molded electromechanical toothbrush it is necessary to leave the blow molded portion of the handle open in at least one end to accommodate the motor, batteries, and drive system components. In this process, the minimum diameter of at least one opening to the blow molded handle must exceed the smallest linear dimension of every component that will be inserted. Such a large opening would be a drawback in a non-electromechanical handle, which has no need to accommodate internal component entry. Further blow molding technology is a high volume manufacturing process. Key challenges for most high volume production is managing variety—like design changes including form, color and decorations. This typically involves switching over certain processes and equipment resulting in equipment downtime. Additionally high volumes of one product design need to be stored or buffered in those cases where different designs want to be combined to be included into one single package. Manual tooth brushes for example are often sold in multipacks that include different colors of the same product form
It is possible to make a hollow blow molded or injection molded handle as one part, and an injection molded brush head as another separate part. Then the hollow handle and a solid or partly hollow brushhead can be assembled by snap fitting or gluing or ultrasonic welding or some other assembly method. There are some prior arts in toothbrushes made of a hollow handle and a hollow or solid brushhead. These prior arts are listed below.
In view of these drawbacks of the prior art, it is an objective of the present invention to provide an improved method for producing a toothbrush in one molding process, which avoids the drawbacks of the prior art.